Fuhua Pure Biology Prelim 2024 Paper 2 Answer

2024 FHSS 6093 Pure Biology Prelim Paper 2 Mark Scheme Section A Qn Mark Scheme Marks Examiner’s Comment 1 a Any 4 • polygalacturonase acts as the lock, pectin acts as the key (active site of polygalacturonase is the keyhole) • The specific 3D shape of the active site of polygalacturonase is complementary to the shape of pectin • Pectin fits and binds to the active site of polygalacturonase to form the enzyme-substrate complex • Activation energy of the reaction is lowered • reaction takes place at the active site to form mono-galacturonate, which leave the active site, which is available to bind with the next substrate molecule 4 Many did not mention fit and bind. Many did not mention that the activation energy is lowered b • Plots accurate; • Axes correct with units; • Best-fit curve • Appropriate Scales Note: if odd scale, penalise 2m for plot + scale; 4 Graph was generally well plotted. A number of candidates use a scale of 10 boxes to 5 days for the y-axis, which caused their graphs to occupy less than half the space.

c Describe: • As concentration of ethylene increased from 0% to 20%, (polygalacturonase concentration increased), average number of days tomatoes took to completely ripen decreased from 20 days to 10 days, (by 10 days). Reject: as concentration of polygalacturonase increased from 0% to 20%...) BOD: if did not calculate difference in number of days decreased 3 Many quoted data for ethylene but referred to concentration of polygalacturonase increasing by those amounts instead, which is not correct.

Explain: • As polygalacturonase concentration increases, there is a higher frequency of effective collision between enzyme and substrate molecules • Higher rate of formation of enzyme-substrate complex and thus products Describe: • As concentration of ethylene increased from 20% to 30%, (polygalacturonase concentration increased), average number of days tomatoes took to completely ripen remained constant at 10 days. BOD: if mistake of describing the % changes as concentration of polygalacturonase changing is made again Explain: • Enzyme concentration is no longer the limiting factor BOD: concentration of ethylene is no longer the limiting factor • Idea of all pectin (substrates) being bound to polygalacturonase (enzymes), so any further increase in enzyme concentration does not increase rate of reaction/ decrease number of days taken. (e.g. when concentration of polygalacturonase increased from 0% to 20%...) Many did not gain credit for the explanation as candidates simply stated that more enzymes breaks down more substrate. d • Ripening bananas release ethylene + ethylene triggers the activity of polygalacturonase • Idea of causing other ripe fruits to overripe, which may spoil/soften the fruits 2 Well done 2 a • G: oesophagus • H: stomach 2 Well done b • E 1 Many candidates indicated F instead. ci • Poor muscular coordination /